1. Field of the Invention
This invention generally relates to integrated circuit (IC) processing and, more particularly, to a method for smoothing a liquid phase epitaxy (LPE) germanium (Ge) film.
2. Description of the Related Art
Ge-on-insulator (GOI) semiconductor devices can be fabricated by depositing Ge, etching the Ge into the desired shape, and depositing an insulator film to encapsulate the Ge features. During a LPE process the Ge melts, but the surface features of the Ge are maintained because the insulator remains solid. Conventionally, the deposited Ge film does not have smooth surface before the LPE process, and the resultant single-crystal Ge film surface retains the pre-LPE process roughness. Active devices (i.e., MOSFETs) built on a rough Ge single-crystal surface face several potential problems. Some of these problems are: (1) gate dielectric reliability; (2) an increase in gate dielectric leakage; and, (3) a decrease in carrier mobility due to surface scattering. While it is possible to smooth the surface after forming the Ge single-crystal film, it is generally acknowledged that the surface quality is improved if the surface can be smoothed prior to the Ge single-crystal formation.
Liu et al. (Yaocheng Liu, Michael D. Deal, and James D. Plummer, “High quality single-crystal Ge on insulator by liquid-phase epitaxy on Si substrate,” Applied Physics Letters, vol. 84, no. 14, pp 2563-2565, 2004) described a technique to form single-crystal Ge on dielectric surface using liquid phase epitaxy. Polycrystalline or amorphous Ge was first deposited on an oxide or nitride surface. A small section of this dielectric layer was etched out to allow direct contact of Ge with the underneath Si surface. A rapid heating cycle caused the Ge film to melt. In the subsequent cooling step, a solidification or crystallization front moved from the region above the single crystal silicon seed and swept across the Ge film. With pre-etched Ge strips formed on an insulator, single-crystal Ge dislocation defects are limited to the Si/Ge seeding window.
In this technique, the Ge strip is enclosed by a dielectric layer. The imprint of the surface feature of the original Ge film remains on the surface after the conversion from polycrystalline or amorphous phase, to the single-crystal phase. In order to form very smooth single-crystal Ge film, the polycrystalline or amorphous Ge film must be very smooth before the LPE process.
A smooth Ge film can be deposited easily by physical vapor deposition (PVD). It would be desirable from a commercial fabrication point of view however, to deposit Ge film using a chemical vapor deposition (CVD) process. It is difficult to deposit a very smooth Ge film by CVD on dielectric layer, due to the low melting temperature of Ge.
FIG. 1A and 1B are drawings depicting the surface roughness of a Ge film formed using a CVD process at 260° C. (prior art). FIG. 1A is an atomic force micrograph of Ge film. The Ge was deposited on a nitride surface by chemical vapor deposition at about 260° C. The Ge thickness is about 170 nanometers (nm). FIG. 1B is line scan analysis of the atomic force micrograph. The Ge grain size is about 100 nm to 200 nm, the peak-to-valley roughness is as high as 50 nm.
FIGS. 2A and 2B are drawings depicting the surface roughness of a Ge film deposited using a CVD process at 520° C. with a 3 nm Si buffer layer (prior art). FIG. 2A is an atomic force micrograph of a Ge film. 320 nm of Ge was deposited on a nitride surface at about 520° C. Even with a 3 nm buffer Si to improve roughness, the surface is still very rough. FIG. 2B is a line scan analysis of the atomic force micrograph. The peak-to-valley roughness is more than 90 nm. A comparison of FIGS. 1 and 2 reveals that the Ge film surface roughness is a more serious problem at higher deposition temperatures. However, the films of FIGS. 1 and 2 are both too rough for the fabrication of electronic devices on a liquid phase epitaxy Ge film.
Conventionally, chemical mechanical polishing (CMP) has been applied to improve the surfaces of polycrystalline silicon, as well as silicon-germanium films in the fabrication of thin-film transistors. CMP processes are widely used in IC industry, particularly to smooth dielectric or metal layers. However, there are no conventional processes for polishing a germanium thin-film. Additionally, there are no reports addressing the issue of smoothing single-crystal germanium films formed through liquid phase epitaxy.